The present invention relates generally to optically transmissive conductors and, more particularly, to optically-transmissive electrodes for electroluminescent devices. In the context of electroluminescent devices, this is a companion to related application Ser. No. 813,928, filed Dec. 27, 1985, concurrently herewith, by Joseph Lindmayer, and entitled "Dielectric for Electroluminescent Devices, and Methods for Making", issued Sept. 15, 1987 as U.S. Pat. No. 4,693,906 the entire disclosure of which is hereby expressly incorporated by reference. The subject invention and the invention to which related application Ser. No. 813,928 is directed are each improvements in the field of electroluminescent devices and, when employed together, result in highly-reliable and bright electroluminescent devices which operate without catastrophic breakdowns.
Electroluminescent devices have a long history, both as lamps and as displays. Earlier development had as its objective the development of a solid state lamp as a light source, typically in the form of a flat panel. More recently, electroluminescence has been employed in flat panel display systems, involving either pre-defined character shapes or individually-addressable pixels in a rectangular matrix.
The basic structure of an electroluminescent device is well known, and comprises an electroluminescent layer sandwiched between a pair of electrodes and separated from the electrodes by respective dielectric layers. Electroluminescence is the emission of light from a polycyrstaline phosphor solely due to the application of an electric field. While various electroluminescent materials are known, one generally accepted is ZnS as a host, with Mn as an activator.
For separating and electrically insulating the electroluminescent layer from the electrodes, a variety of dielectric materials have been proposed and employed, a subject to which the above-identified companion application Ser. No. 813,928 is directed.
The electrodes differ from each other, depending upon whether it is the "rear" or the "front" (viewing) side of the device. A reflective metal, such as aluminum, is typically employed for the electrode on the "rear" side of the device, and a relatively thin optically transmissive layer of indium tin oxide (ITO) is typically employed for the electrode on the "front" side of the device. In lamp applications, both electrodes take the form of continuous layers, thereby subjecting the entire electroluminescent layer between the electrodes to the electric field. In a typical display application, the "front" and "rear" electrodes are suitably patterned so as to define row and column electrodes. Pixels are thus defined where the row and column electrodes overlap. Various electronic display drivers are well known which address individual pixels by energizing one row electrode and one column electrode at a time.
While seemingly simple in concept, the development of electroluminescent devices has met with many practical difficulties. Very generally, these practical difficulties arise from two factors. First, the devices are thin-film devices where even a small defect in a particular layer can cause a failure. Second, these thin-film devices are operated at relatively high voltages, typically ranging from 100 volts to 400 volts peak-to-peak. In this regard, electroluminescent devices are perhaps unique among solid state electronic devices in that the ZnS electroluminescent layer is operated beyond its dielectric breakdown voltage, and thus conducts, while the thin-film dielectric layers on either side are required to stop the conduction.
Manifestly, even a small defect can lead to catastrophic failure, and this has indeed been a problem with the prolonged application of large electric fields, accompanied by high temperatures during operation.
The present invention is particularly directed to the "front" optically-transmissive electrode, which typically comprises a layer of indium tin oxide (ITO) approximately 200 nanometers (200.times.10.sup.-9 meters) in thickness deposited directly on a glass substrate. After the ITO layer is deposited, the glass substrate and the ITO layer are heated to approximately 500.degree. C., which causes the ITO to become electrically conductive. Electrically-conductive ITO-coated glasses are commercially available as a stock material.
ITO layers having resistivities of from 20 to 1000 ohms per square are typical. It is known that an electrode layer of greater resistivity, for example in the range of 4000 to 6000 ohms per square, is advantageous in that the higher resistance mitigates the effects of a localized incipient failure in the dielectric layers by limiting the current which can flow. Thus, a resistive electrode can limit the propagation of a failure, the propagation of a failure typically being manifested by local melting of the dielectric and electrodes. With a relatively higher resistivity electrode layer, the device can continue to operate with minor failures in the dielectric which otherwise would result in catastrophic breakdown and device failure.
However, to achieve such high resistivity with indium tin oxide requires an ultra-thin layer, less than 100 angstroms (100.times.10.sup.-10 meters) thick. This, then, introduces other drawbacks. In particular, such an ultra-thin ITO layer is unable to reliably carry the lamp currents involved in operation of the device. Such a thin ITO layer has a tendency to strongly change its conductivity, and/or disconnect and burn up.